late 14c., an astrological term for the situation of two heavenly bodies exactly across from one another in the heavens, from Old French oposicion (12c.) or directly from Latin oppositionem (nominative oppositio) "act of opposing, a placing against," noun of action from past participle stem of opponere "set against" (see opponent). Meaning "that which is opposite something else" is from 1540s; meaning "contrast, antagonism" first attested 1580s; sense of "political party opposed to the one in power" is from 1704. Related: Oppositional.